Method for monitoring a welding process during the welding of studs and device for carrying out said method

ABSTRACT

The invention relates to a method of monitoring the welding operation in a stud welding process, wherein a part ( 19 ) to be welded is immersed into the melt by moving the part ( 5 ) towards a workpiece ( 17 ), with a physical quantity depending on the force required for moving the part ( 5 ) being measured at least during the phase of immersion of the part ( 19 ) to be welded into the melt and the measured gradient of the physical quantity plotted against time being compared with a predetermined set gradient. Furthermore, the invention relates to a stud welding device for carrying out such a method.

[0001] The invention relates to a method of monitoring the weldingoperation in a stud welding process and a device for carrying out themethod with the features of the preambles of patent claims 1 and 6.

[0002] When carrying out a stud welding process, in order to obtain thebest possible welding quality, it is necessary to observe the set valuesof relevant welding parameters with corresponding accuracy. In thestroke ignition method and in similar methods which the presentinvention relates to, the parameters welding current, welding time,length of projection, stroke or lifting-off distance when drawing thearc, and dampening during immersion in particular are decisive for thequality of the welding connection between the stud and the workpiece.

[0003] Measuring and monitoring the electric parameters, namely, thewelding current and the welding voltage, has been known and commonlyapplied for quite a long time, but particularly in case of the strokeignition method, observing the mechanical parameters such as the lengthof projection, the lifting-off distance and immersion dampening, hasonly been made possible by pre-adjusting these parameters in the studwelding device or the corresponding welding head.

[0004] What is disadvantageous here, however, is that the movement ofthe stud of known stud welding devices inserted into the welding head isnot monitored and thus deviations of the movements from predeterminedset values are not detected. This means that a negative influence on thewelding quality resulting from the fact that a predetermined length ofprojection or lifting-off distance was not observed, or deviations fromthe set value of dampening during immersion are not detected.

[0005] In order to solve this problem in the tip ignition weldingprocess, the German patent DE 41 24 511 C1 proposes that the actualvalue of the immersing speed of the welding stud into the welding poolthat is created at the point of welding of the workpiece be measured andcompared with a set value for the speed obtained empirically. Deviationsof the actual value from the set value which are outside a predeterminedrange of tolerances then show the insufficient quality of the weldingconnection that has been completed. As the immersing speed is thedecisive parameter for the welding quality in the tip ignition weldingprocess, the document DE 41 24 511 C1 proposes that this value bedetermined by measuring the time which the welding stud requires inorder to cover a short distance Δs immediately before it is immersedinto the melted mass of the workpiece.

[0006] In the stroke ignition welding process, however, the quality of awelding connection does not only depend on the immersing speed, but alsoon the dampening during immersion, which means that what is decisive isthe time gradient of the movement of the stud or the time gradient ofthe stud speed from the point of time when it is immersed into the melt.Furthermore, it is desirable to monitor the observance of thepredetermined parameters “length of projection” and “lifting-offdistance”, because they have an influence on both the stud speed duringthe phase when it is immersed into the melt and on the behaviour of theelectric arc.

[0007] From the documents DE 39 29 669 A1 and DE 43 07 325 A1, studwelding devices are known, respectively, whose aim it is to effect amovement of the moving part assembly (which comprises all moving partsthat cooperate to perform the actual welding movement such as the studholding device, the charging head, the feed cylinder, the feed piston,etc.) or of the stud holding device as exactly as possible and in linewith a predetermined course. For this purpose, in the device accordingto DE 39 29 669 A1, a control magnet is connected with a path measuringdevice and an electronic control in order to thereby determine theactual value of the regulating distance of the armature of the controlmagnet, to compare it with a set value and to effect control of themovement of the armature with which the moving part assembly isconnected. In the device according to the document DE 43 07 325 A1, themoving part assembly or the stud holding device of the welding gun ismoved with the aid of a lifting magnet which acts as a controlled brakeso that no additional damping units or devices are required. What isdisadvantageous in these devices, however, is that insufficient weldingquality of completed welds is not recognizable if—for whateverreasons—the course of welding deviates from an ideal or predeterminedcourse.

[0008] For this reason, in the document DE 43 14 528 C2, it is proposedto continuously measure the time gradient of the movement of the studtowards the workpiece, at least during the entire phase of movement whenthe stud is immersed into the melt of the workpiece. Hereby, deviationsof the measured course of movement or of the time gradient of the speedof movement determined therefrom from a predetermined set value for thecourse or the gradient can be detected.

[0009] In spite of this well-tried method, in practice, there was anincreased demand for more exact and more detailed information about thequality of a weld, particularly in case of aluminium welding.

[0010] Thus, the object of the invention is to provide a method ofmonitoring the welding operation in a stud welding process whichimproves the control of the quality of a welding connection completedand which increases the sensitivity or the exactness of quality control.Furthermore, it is an object of the invention to provide a device forcarrying out this method.

[0011] The invention achieves this object with the features of patentclaims 1 and 6.

[0012] With analogous or digital (continuous or, with predeterminedsampling frequency, discrete) detection or measurement of the forcerequired—or of a physical quantity clearly mathematically dependentthereon—for the movement of the part (such as the moving part assemblyof a welding gun or the welding axle of a welding head) at least duringthe phase of immersion of the part to be welded or the stud into themelt, it is now possible to monitor the process of immersion in a moredetailed way; here, even such parameters as the viscosity of the meltetc. are included in the quality information about the weld, as theforce required increases with reduced viscosity. For this purpose, inorder to assess the quality of the weld, the measured gradient of theforce required is compared with a predetermined set value for thisgradient. In this way, actual curves can be determined which, even incase of little changes of the path taken, provide much more sensitiveand thus more detailed or more exact information about this importantphase of a welding operation compared to path measurement (a curveplotting path against time). In particular, it is possible to assess thequality of a weld by making a comparison with a set curve determined,with potentially additional tolerance, and to make a reliable statementwhether the weld obtained meets the predetermined demands.

[0013] Advantageously, the force required can be detected via a signalthereon dependent, namely, the power consumption or input of theelectric drive of the movable part or the moving part assembly (weldingaxle). Of course, it is conceivable to determine the force required viaa signal thereon dependent which is obtained by means of other sensors(such as mechanical, optical or electrical ones) and by a correspondingevaluation.

[0014] Advantageously, a welding device with an electric drive,particularly a servoelectric drive, does not require an additionalsensor because a signal, namely, the electric power consumption of thedrive, already exists here, and this signal only has to be transmittedto a corresponding evaluation and control unit. Here, even alreadyexisting stud welding devices can be retrofitted in a simple manner, astapping off the current and/or the voltage of the electric drive is notvery complicated to effect and old measuring and evaluation units can bereplaced or retrofitted, or new measuring and evaluation units can beadded.

[0015] Of course, it is sufficient to determine and to monitor onesignal or parameter only, which the force required is unambiguouslydependent on. For example, if the voltage is constant, it is sufficientto determine or measure the power consumption of the electric drive, asthe force directly depends on the power consumption and, in case ofconstant voltage, the latter depends on the current consumption orinput.

[0016] In a preferred embodiment of the invention, if predeterminedtolerances are exceeded, a failure signal can be produced and thefailure can be indicated optically or acoustically. In hand-held tools,for example, a failure can advantageously be indicated at the weldinghead or at the welding gun by means of an indicating unit or at anotherarbitrary place of the welding device, with even a central display beingpossible.

[0017] In another embodiment of the invention, the set course orgradient and/or the predetermined tolerances may depend on the energysupplied during the welding operation, particularly the welding current,and other parameters such as the thickness, the material of theworkpiece, the type of stud (thickness, material, length, shape,cross-section) etc. Set courses or gradients of this kind may bedetermined mathematically or empirically in tests and may be stored in adata storage medium, particularly of the measuring and evaluation unit,in such a way that they can be chosen and fetched from the storagemedium.

[0018] In a further embodiment of the invention, the measured timegradient of the force is logged and/or stored so that the informationabout the quality of the weld is not only available during or shortlyafter the welding operation (for example, using a temporary storagemedium), but is advantageously stored at least in a spot-checking-way oras an entire data set in order to make it possible to make statementsabout individual welds or all welds in retrospect. For example,particularly in case of welds for which safety is an important aspect,the quality of the weld can be checked or proved subsequently with theaid of a welding protocol.

[0019] In addition, it is conceivable to determine the movement of thepart to be moved, particularly the moving part assembly or the weldingaxle (as suggested in document DE 43 14 528 C2, for example) with a pathmeasuring unit and to evaluate it with respect to the quality of theweld as a further parameter (depth of immersion, lifting-off distanceetc.) in addition to the force applied and to use it for controlling themovement.

[0020] It is of course also possible to use the (negative) accelerationor the braking of the part to be moved, particularly during the phase ofimmersion, as a further parameter to improve the assessment orevaluation of the quality of the weld additionally or instead of theparameters described above. The negative acceleration or an accelerationcurve can be calculated via a path measuring unit and by making acorresponding differentiation of the path measured during a certainperiod of time, or it can advantageously be determined in a simple andquick manner directly during the movement, particularly during the phaseof immersion, with the aid of an acceleration sensor.

[0021] Moreover, it is conceivable to classify the determined actualcourses or gradients potentially together with other parameter curves,such as the curve plotting path against time, the curve for the weldingcurrent, the curve for the electric arc voltage, the acceleration curve,etc. of a welding operation, particularly the phase of immersion, andwith parameters such as the lifting-off-distance, the type of workpiece,the type of stud, etc. With the aid of such a classification, it isadvantageously possible to determine set curves for a process controlwhich can be used for controlling welding operations. For this purpose,the measuring and evaluation unit may additionally be configured as acontrol unit, or the signal evaluated may be transmitted to anindependent control unit which controls the drive of the welding axle orthe moving part assembly and/or the arm of a robot in dependence on thesignal evaluated (open-loop control) and effects control (closed-loopcontrol) in accordance with set values or curves which can bepredetermined (as compared to actual values detected, respectively).

[0022] Further embodiments of the method according to the invention andof the device according to the invention are apparent from thesubclaims.

[0023] The invention is now described in greater detail with the aid ofan embodiment shown in the drawings, in which:

[0024]FIG. 1 shows a schematic view of a stud welding device accordingto the invention;

[0025]FIG. 2 shows a diagram of different signals during a weldingoperation carried out with a stud welding device according to FIG. 1 and

[0026]FIG. 3 an enlarged partial view of the diagram according to FIG.2.

[0027]FIG. 1 shows a stud welding device 1 which comprises a studwelding head 3 which in turn consists of the actual welding axle 5 andan electric drive 7, particularly a servoelectric drive such as a linearmotor, in order to move the welding axle 5 in the axial direction (arrowI in FIG. 1).

[0028] The entire welding head 3, which is substantially embodied in agenerally known way as a welding head that is suitable for carrying outthe stroke ignition welding process, may be arranged at the arm 9 of arobot, as indicated in FIG. 1, with which the welding head 3 can bemoved in one or several three-dimensional axes.

[0029] Both the drive 7 for the welding axle 5 and a drive 11 for thearm 9 of the robot are controlled by means of an evaluation and controlunit 13.

[0030] The evaluation and control unit 13 can additionally take over (ina way not shown) the further controlling operations of the welding head3 inasmuch as the control of the welding current is concerned. As thepresent invention substantially relates to a movement of the weldinghead 3 or of a stud holding device 15 of the welding axle 5, theillustration of further components of the stud welding device 1 wasrelinquished.

[0031]FIG. 1 also shows a workpiece 17 onto which a part (stud) 19 to bewelded which is held in the stud holding device 15 is to be welded. Theworkpiece 17 is electrically contacted with the evaluation and controlunit 13. This is not only necessary to make the welding current flowduring the welding operation, but it is also advantageous for detectingan electric contact 32 between the stud holding device 15 or the part 19to be welded and the workpiece 17.

[0032]FIG. 1 furthermore shows that parts 19 to be welded can besupplied to the welding axle 5 of the stud welding head 3 by means of anautomatic feeding device which is not shown in greater detail. This isdone by means of a supply channel 21 in the front portion of the weldinghead 3 which is connected with the automatic feeding device in acommonly known way using a tube (not shown).

[0033] Feeding or supplying a part 19 to be welded is effected usingblast air, whereby the part to be welded is moved into the front portionof the stud welding head 3 via the supply channel 21. In this position,the part to be welded substantially already lies on the axis of thewelding axle 5 or the stud holding device 15. Then, the part 19 to bewelded is pushed through the stud holding device into the front portionthereof by means of a charging pin 23 to assume an axial position inwhich it is possible to weld the part 19. For this purpose, the part 19of course has to project from the face of the stud holding device by apredetermined length of projection d. The charging pin 23 is acted uponby a drive 25 which is only shown schematically in FIG. 1 and which isusually embodied as a pneumatic drive. The drive 25 makes it possible tomove the charging pin 23 between a front position in which the face ofthe charging pin 23 acts upon the backside of the part 19 to be welded,and a withdrawn position in which the front portion of the charging pin23 is withdrawn to such an extent that the supply channel 21 forsupplying a part to be welded is exposed. The drive 25 is controlled bythe measuring, evaluation and control unit 13, as well.

[0034] In the following, the stud welding method according to theinvention is explained in greater detail with the aid of a device shownin FIG. 1.

[0035] In the diagram in FIG. 2, typical curves of an entire weldingoperation are illustrated, with the following information beingprovided:

[0036] Curve 1: Current gradient of the servoelectric drive 7 without asmoother

[0037] Curve 2: Gradient of the electric arc voltage

[0038] Curve 3: Current gradient of the servoelectric drive 7 with asmoother (RC)

[0039] Curve 4: Course of the path travelled

[0040] Curve 4 shows a curve plotting path against time which is typicalof a stud welding operation working with the stroke ignition method.Here, the welding axle 5 is moved from a starting position (1^(st)quadrant) towards the position of the workpiece 17 as far as thetouching point 30 (2^(nd) quadrant to the middle of the 4^(th)quadrant). Here, the lifting-off-distance 32 and the depth of immersion34 are calculated or determined. When the tip of the stud 19 is liftedoff the workpiece, the electric arc voltage (curve 2) increases fromzero (switching on the pre-current) and reaches its maximum in thecourse of the adjustment of the lifting-off-distance 32. Furthermore,the welding axle 5 is moved towards the workpiece 17 together with thestud 19 again, and the tip of the stud 19 is immersed into the meltproduced by the electric arc and is moved further until a depth ofimmersion 34 which can preferably be predetermined is reached.

[0041] Further details not relating to the phase of immersion, such asovershooting, bouncing and the like, are not discussed here, as they arenot of great importance to the method according to the invention.Further effects or deflections which are not important to theinvention—apart from errors of measurement—are based on PWM control(pulse width modulation) of the servoelectric drive 7 as well as apotential transient response of the path control (lifting-off distance,depth of immersion, etc.). In addition to effects caused by control(stopping), a certain amount of overshooting or undershooting in thecurrent gradient of drive 7 is also caused by the inertia of masses ofthe parts moved (basically the welding axle 5, the stud 19, the studholding device 15) and, as the case may be, gravity (in case the weldingposition is not horizontal, such as during overhead welding); thus,after the inertia of masses has been surmounted (increase in currentwith overshooting), the part to be moved or the welding axle 5 movesfurther automatically and may cause a certain “generator effect”(decrease in current up to negative under-shooting).

[0042]FIG. 3 shows an extract of the 5^(th) and 6^(th) quadrants (leftand right quadrant of the ordinate) according to FIG. 2 which has beenextended in the direction of the absciss (axis of time), which will bediscussed in greater detail subsequently.

[0043] The dashed line drawn into the diagram as an optical guidelinenear curve 4 (gradient or course of the path travelled plotted againsttime) shows that the tip of the stud is immersed into the melt at thepoint of time T_(E1). As will be apparent from curve 3 and particularlycurve 1, at this point of time, the current consumption of theservoelectric drive 7 increases. In one example of a control operation,the current may be limited to a specific current intensity (such as 50%or 70% of the maximum current intensity) after a speed that can bepredetermined has been reached (set value), so, at the point of time ofimmersion into the melt, the current consumption increases because of astopping or slowing down effect so that the set value for the speed isreached again.

[0044] In the course of further immersion into and penetration of themelt, however, the current consumption may drop again because of effectssuch as a higher viscosity of the melt after the cooled surface has beensurmounted or the resistance by the surface tension has been surmounted.When the tip of the stud further penetrates the melt, however, thecurrent consumption may increase again at the point of time T_(E2)because of increasing resistance resulting from decreasing viscosity ofthe melt (cooling and depth of the melt) until the depth of immersion 34is reached and the current for drive 7 can be switched off.

[0045] In practice, it has been observed that this curve, particularlythe first rise (between T_(E1) and T_(E2)) and the potential drop afterimmersion into the melt, is particularly telling with respect to thequality of the weld, because the condition of the melt, particularly theviscosity thereof, contributes to the course of the curve (curve 1) as adecisive parameter. Thus, according to the invention, set curves withpotential tolerances can be created by calculation or in several tests,e.g. by obtaining the mean value or average value, preferably independence on the workpiece, the type of stud, the welding current andthe welding position, within which an actual gradient of a weld withsufficient quality has to range.

[0046] Here, it is advantageously possible to define a threshold valuefor the drive current; as soon as this value has been reached—with apredetermined additional delay, as the case may be—, the drive currentis switched off. Hereby, damage or a change of the position such as thedisplacement of the workpiece and/or the welding device can be avoided.

[0047] Of course, the curves illustrated only constitute examples, andtheir course largely depends on manufacturing parameters (such as thetype of stud, the workpiece, the type of weld etc.). However, tests haveshown that characteristic (set) curves may be obtained for differentconditions, which make it possible to make a clear assessment of thequality of a weld.

[0048] In a further embodiment of the invention, different events suchas “immersing T_(E1)” may be detected not by an additional pathmeasuring unit but by evaluating curve 3 or, preferably, curve 1 and bysearching for concise events such as the first positive slope after thelifting-off-distance has been reached or after continued currentconsumption (for lifting-off distance and drop), or the course andduration of the corresponding curve segments, etc.

[0049] In addition, it is conceivable to use the result of evaluationfor controlling (open- or closed-loop control) the drive 7, preferablyby means of digital control, so that predetermined set values or setcurves including tolerance can be defined and that the quality of theweld can be assessed in accordance with the invention.

[0050] For example, in case of a large number of welds made, it ispossible hereby to examine only those welds for their quality inadditional operations which have been found—with the aid of directfailure indication or a corresponding protocol—to be potentially faulty,i.e. beyond the tolerance of the set curve, and which have thereforebeen found not to be sufficiently durable, for example. Particularly incase of aluminium welding, manufacturing time and costs are reduced to aconsiderable extent, because here all welds had to be subsequentlyexamined for durability—for example manually using a cordless screwdriver—by now.

[0051] Furthermore, it is conceivable to examine and classify the actualcourses or gradients detected, potentially together with furtherparameter curves, such as the curve for the path plotted against time,the curve for the welding current, the curve for the electric arcvoltage, etc. of a welding operation, particularly the phase ofimmersion, and parameters such as the lifting-off-distance, the type ofworkpiece and stud etc. with respect to characteristic properties whichare decisive for the quality of a weld. With the aid of suchclassification, it is advantageously possible to determine set curvesfor a process control (open- or closed-loop control) with which weldingoperations can be controlled. For this purpose, the measuring andevaluation unit may additionally be configured as a control unit, or thesignal evaluated may be supplied to an independent control unit whicheffects control of the drive of the welding axle or the moving partassembly and/or the arm of a robot in dependence on the signal evaluated(open-loop control) and (in comparison with actual values detected)effects control in accordance with set values or curves that can bepredetermined (closed-loop control).

[0052] Of course, the method according to the invention and the deviceare not limited to the embodiment shown, but can be used in arbitrarystud welding methods (stroke ignition method, tip ignition method, etc)with arbitrary stud welding devices (welding head of an automaticmachine or of the arm of a robot, welding gun, etc.). Here, the term“stud” according to the invention is not to be interpreted in a narrowsense, but in a broad sense, which means that any arbitrary part to bewelded which is to be welded to a workpiece with one end or one sidethereof is included in this term.

1. A method of monitoring the welding operation in a stud weldingprocess, in which a) a part (19) to be welded is immersed into the meltby moving the part (5) to wards a workpiece (17) characterized in thatb) a physical quantity depending on the force required for moving saidpart (5) is measured at least during the phase of immersion of said partto be welded (19) into the melt and c) the measured gradient of thephysical quantity plotted against time is compared with a predeterminedset gradient.
 2. A method according to claim 1, characterized in that,as the physical quantity, the power consumption (curve 1) of an electricdrive (7) for moving said part (5) is measured.
 3. A method according toclaim 1 or 2, characterized in that a failure signal is produced ifpredetermined tolerances are exceeded, and the failure is indicatedpreferably optically or acoustically.
 4. A method according to one ofthe preceding claims, characterized in that the set course or gradientand/or the predetermined tolerances depend on the energy that issupplied during the welding operation.
 5. A method according to one ofthe preceding claims, characterized in that the measured time gradientof the physical quantity is logged and/or stored.
 6. A stud weldingdevice (1) for carrying out the method according to one of the precedingclaims, a) comprising a welding head (3) with a controllably movablepart (5) at which a stud holding device (15) for receiving a part (19)to be welded is arranged, characterized in that b) said stud weldingdevice (1) comprises a measuring and evaluation unit (13) to which, inorder to detect the force required for moving said controllably movablepart (5), a signal is supplied which depends on the force required, c)so that, in order to assess the quality of the welding connectioncompleted, said measuring and evaluation unit (13) detects the timegradient of the signal at least during the entire phase of movementtowards immersion of the stud in the melt and compares the time gradient(curve 1) of the signal with a predetermined set gradient.
 7. A studwelding device according to claim 6, characterized in that said weldinghead comprises an electric drive (7) for moving said part (5) and thatthe power consumption of said electric drive (7) serves as the signalthat depends on the force required.
 8. A measuring and evaluation unit(13) for a stud welding device according to claim 6 or 7 for carryingout a method according to claims 1 to 5, characterized in that aphysical quantity depending on the required force of said drive (7) canbe measured by means of the measuring unit and compared with a set curvewhich can be predetermined.